The present invention relates to the field of bone implants, particularly to cranial implants designed to fill a bone defect in the skull of a human or a mammal.
Treatment of bone defects of the skull is now a major problem in maxillofacial surgery and in neurosurgery. The cranial defects may have various origins and occur especially following a developmental anomaly, a bone disease or trauma that resulted in fractures of the skull. They may also be the consequence of a neurosurgical procedure, such as a decompressive craniectomy performed in the case of post-traumatic cerebral edema, or a surgery of extraction of a brain tumor.
These bone defects are repaired during a reconstructive skull surgery, called cranioplasty, during which a bone implant destined to fill the bone defect is inserted and fixed in the skull.
The ideal material for the cranioplasty implants must be resistant, biocompatible, to eliminate the risk of inflammation, rejection and infection, and must be able to integrate with the living bone structure to ultimately form a part thereof. The integration of the implant within the native tissue proceeds via a migration process of bone cells, the osteoblasts, into the implant, which will contribute to the formation of a new bone tissue.
In this regard, several types of materials have been used to make these implants. Bone itself has thus been commonly used by performing an autograft from other bone structures of the patient, such as ribs. This solution is not without drawback: it presents risks of morbidity at the donor site; the material is only available in limited quantity and is subject to conservation challenges.
Artificial bone implants provide an interesting alternative to the use of bone grafts. In particular, metallic or plastic implants have been used. However, these implants often lack biocompatibility, and have a low osteoconductivity, which diminishes their integration ability within the existing bone tissue.
Ceramics are also advantageous materials for the manufacture of cranioplasty implants. Thus, hydroxyapatite or calcium phosphate implants are known, materials selected for their suitable biocompatibility and osteoconductivity.
WO2005/094730 A1 describes a manufacturing method of a porous ceramic cranial implant. After acquiring a three dimensional image of the skull of the patient, the bone defect is modelled by computer-aided design. Then, by rapid prototyping, an implant prototype is made, which will be used to make a calcium sulphate, resin or silicone rubber mold. Finally, the mold is used to manufacture the porous ceramic implant.
Ceramics such as hydroxyapatite or tricalcium phosphate have interesting properties for manufacturing implants. Their chemical composition, based on calcium phosphate, is close to that of the bone matter, and imparts them with a good biocompatibility. In addition, the ceramics can be manufactured porous and therefore have good osteoconductivity which allows the implants to integrate well with the bone tissue.
However, one of the drawbacks of ceramic implants is their fragility both during the manufacturing process and on the finished workpiece. Cranial implants are workpieces that frequently have a cap shape with a relatively small thickness compared to their surface. These implants are thus subjected to large internal stresses which exert in particular during the heat treatments to which the workpiece is submitted during its manufacture: debinding and sintering. They therefore lead to a distortion of the implant, which may render it unusable, since unsuited to the shape of the bone defect. They can also cause the occurence of areas of fragility of the implant likely to promote its rupture. These drawbacks make it difficult or even impossible to manufacture large size ceramic implants, which limits the use of this material to the repair of small size bone defects.